Revolving rivers are a surprising, uncommon way of

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

pile growth that can be found in a few sands around the world, but has been studied in detail only for one

Cuba Cuba, officially the Republic of Cuba, is an island country, comprising the island of Cuba (largest island), Isla de la Juventud, and List of islands of Cuba, 4,195 islands, islets and cays surrounding the main island. It is located where the ...

n sand from a place called Santa Teresa ( Pinar del Rio province).

Description

When pouring "revolving" sand on a flat surface from a fixed position, the growth of a conical pile does not occur by the common

avalanche An avalanche is a rapid flow of snow down a Grade (slope), slope, such as a hill or mountain. Avalanches can be triggered spontaneously, by factors such as increased precipitation or snowpack weakening, or by external means such as humans, othe ...

mechanism, where sand slides down the pile in a more or less random fashion. What happens in that a relatively thin "river" of flowing sand travels from the pouring point at the apex of the pile to its base, while the rest of the sand at the surface is static. In addition, the river "revolves" around the pile either in clockwise or counter-clockwise directions (looking from top) depending on the initial conditions. Actually the river constitutes the "cutting edge" of a layer of sand that deposits as a helix on the conical pile, and makes it grow. For small sandpiles, rivers are continuous, but they become intermittent for larger piles.

History and state of the art

The phenomenon was observed first by E. Altshuler at the

University of Havana The University of Havana (UH; ) is a public university located in the Vedado district of Havana, the capital of Cuba. Founded on 5 January 1728, the university is the oldest in Cuba, and one of the first to be founded in the Americas. Originall ...

in 1995, but at the time he assumed that it was well known, and temporarily forgot about it. In 2000, being at the

University of Houston The University of Houston (; ) is a Public university, public research university in Houston, Texas, United States. It was established in 1927 as Houston Junior College, a coeducational institution and one of multiple junior colleges formed in ...

, he told K. E. Bassler, who showed a vivid interest in the matter. Embarrassingly enough, Altshuler was unable to demonstrate it before Bassler using a random sand from Houston, so he had to send him a video from

after his return to the island. Once the existence of the strange phenomenon was confirmed for everyone, E. Altshuler and a number of collaborators performed a systematic study in

Havana Havana (; ) is the capital and largest city of Cuba. The heart of La Habana Province, Havana is the country's main port and commercial center.silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

grains with an average

grain size Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which ...

of 0.2 mm approximately and no visible special features regarding grain shape. But in spite of its apparent simplicity, many puzzles still remain. For example, after many experiments one batch of sand may stop showing revolving rivers (just as singing sand eventually stops singing), which suggests that the decay is connected to certain properties of the surface of the grains that degrade by continued friction. Videos of the effect are available on YouTube.

References

{{Reflist Physical phenomena Soft matter Granularity of materials Geomorphology Complex systems theory Condensed matter physics Sand